Redefine result() method to return List

Many bignum tests have multiple calculated result values, so return
these as a list, rather than formatting as a string.

Signed-off-by: Werner Lewis <werner.lewis@arm.com>
diff --git a/scripts/mbedtls_dev/bignum_common.py b/scripts/mbedtls_dev/bignum_common.py
index c340cd5..88fa4df 100644
--- a/scripts/mbedtls_dev/bignum_common.py
+++ b/scripts/mbedtls_dev/bignum_common.py
@@ -108,10 +108,12 @@
         self.int_b = hex_to_int(val_b)
 
     def arguments(self) -> List[str]:
-        return [quote_str(self.arg_a), quote_str(self.arg_b), self.result()]
+        return [
+            quote_str(self.arg_a), quote_str(self.arg_b)
+        ] + self.result()
 
     @abstractmethod
-    def result(self) -> str:
+    def result(self) -> List[str]:
         """Get the result of the operation.
 
         This could be calculated during initialization and stored as `_result`